Fuel cells can convert chemical energy resulting from the oxidation of fuels directly into electrical energy and are considered to be one of the most important clean energy conversion devices. Currently, low-temperature fuel cell research is quite active particularly in automobile-related fields, backup power, portable and mobile power source because low-temperature fuel cells have advantages in improved fuel efficiency, reduced emission, and more environmentally friendly compared to their internal combustion engines counterpart.
Proton exchange membrane fuel cells (PEMFCs, including direct methanol/ethanol fuel cells, DMFCs) are characterized in that a wide operating temperature range from −20° C. to 180° C. (depending on the solid electrolyte properties), quick start-up and response, and high output power density which allows PEMFCs system to be readily smaller and lighter. PEMFCs are very suitable to be the power source for an automobile, unmanned aerial vehicle (UAV), auxiliary power units (APU), mobile market, portable, small stationary power applications, or a power supply for a small cogeneration system such as a combined heat and power (CHP) system.
The core of PEMFCs is membrane-electrode assembly (MEA) which is composed of a solid electrolyte sandwiched in between two catalytic electrodes. The electrode used generally contains a catalyst layer, a macro-porous layer and a backing layer. The catalyst layer can be fixed directly on solid electrolyte, or supported on backing layer. Currently, PEMFCs employ noble metals especially scarce platinum or its alloy supported on carbon materials as electrode catalysts to promote the reactions of fuel electro-oxidation and oxygen reduction. In current hydrogen-fed PEMFCs, around 75% of precious metal is used as a cathode catalyst to accelerate the sluggish oxygen reduction reaction. Hence, it is imperative to reduce or eliminate the use of platinum in the cathode, which would lead to a more affordable fuel cell system as a whole and is made possible for large volume commercialization.
In order for the fuel oxidation and oxygen reduction reactions in a fuel cell to occur at desired electrochemical kinetic rates and potentials, highly active and durable electro-catalysts are required. Due to the high catalytic nature of platinum and its chemical stability, platinum and platinum alloy materials, supported or unsupported, are preferred as electro-catalyst for the anode and cathodes in low-temperature fuel cells.
Generally, to reduce the impact of costly platinum, conductive materials such as carbon and its derivatives are used to support platinum-based catalysts, which can also help to improve the stability and the dispersion of noble metals. There are two ways to reduce the electrode catalyst cost, which would lead to a reduction in the cost of a PEMFC stack as a whole. One way is to employ non-precious metal catalysts or non-metallic catalysts, which is more attractive and interesting. Another way is to decrease the Pt loading to increase the cost-effectiveness. Although, researchers are striving to improve the quality of the catalysts, the current use of non-precious metal catalysts is still limited by the limited activity in the acidic environment of solid polymer proton conducting electrolyte. Due to this reason, non-platinum catalysts that were developed have little opportunity to replace platinum-based catalysts at least not in the foreseeable future.
Alloying platinum with various less-expensive materials is one of the possible avenues to either reduce the amount of platinum required or increase the total activity of electrocatalysts, or both. Recently, platinum-based nanostructures such as platinum thin shells capped cheap metallic cores or metal-oxide cores were also suggested to reduce the use of platinum and increase the catalytic activity. Successive reduction procedures or in-situ displacement reactions are always used to achieve the core-shell nano-catalysts. Generally, it is believed that the interaction between the shell metal and core metals/oxides can enhance the activity and durability to some extent.
The reduced noble metallic particle sizes, on the other hand, can produce more surface active sites and can therefore increase the available reaction sites in the electrode's catalyst layer or the so-called three-phase boundaries. Various methods have been reported to prepare supported Pt or Pt-based catalysts with high metallic loading.
U.S. Pat. No. 7,713,902 reported a procedure in which lauric acid was used as a surfactant, and mesoporous alumina as template to prepare highly dispersed platinum. The resulting platinum particle size achieved with this method was ranging from 1.0 to 2.4 nm in diameter. In this patent, the procedure described needs to be carried out in butanol-water solution, and the catalyst activity was enhanced compared to the commercial E-TEK catalyst.
In U.S. Pat. No. 5,759,944, metals were deposited by suspension of the support material in water, subsequently hydrolysis or precipitation of the corresponding noble metal salts and non-noble metal salts with aqueous reducing agent such as formaldehyde was carried out. In this procedure, a heat treatment was needed for the synthesized samples such as Pt, PtNi and PtNiAu in an inert or reducing atmosphere.
U.S. Pat. No. 6,689,505 employed a home-made carbon black containing an H-content of greater than 4,000 ppm to synthesize the supported platinum catalyst. The resulting nanoparticle size was around 4.4 nm, which was carried out in aqueous solution at pH of 9. It was claimed that the H-content was helpful to the deposition of particle and activity improvement.
Support pre-treatment or functionalization can also play an important role in reducing the particle size of noble platinum and immobilizing metallic nanoparticles on support surface. US Patent Publication No. 2012/0149545 demonstrated that ammonia treatment of carbon carriers can reduce platinum particles to 1.28 nm on treated carbon powder from 2.26 nm on non-treated carbon powder with a 20 wt % platinum loading. The big increase in surface area results in larger electrochemical surface areas of platinum and the big improvement of activity.
Lin et al. presented in PCT Patent Publication No. 2008/048192 a method to functionalize carbon materials as carriers to support noble nanoparticles. The typical functional groups introduced in that invention were mainly the oxygen-containing radicals such as: —COOH (carboxyl), —CHO (aldehyde), —CO— (carbonyl), —O— (oxo) and —OH (hydroxyl). These surface oxygen-containing radicals can assist in immobilization of metallic precursors and therefore the dispersion of reduced metal particles. With this method, nanoparticle sizes of noble metal can be reduced with sparse surface particle density.
Therefore, there remains a need to provide for improved methods for preparing noble metal nanoparticles suitable for use as a catalyst for fuel cell applications, for example.